Coating of Medical Products
Bone Screws
Several screw-dependent factors such as material of the screw, the screw diameter and length, the length of the threaded part, and the shape of the threads affect the forces experienced by the screws. Also the properties of the bone, such as the thickness of cortices and the density of bone, influence the forces required in screw installation and pullout. Screw failures during installation increase operation time and require the drilling of several holes in bone. Since drilling a hole into a bone reduces its torsional strength by 10-40% depending on the screw/bone diameter rate, it is essential to avoid screw failures during operations. Although current biomaterials are compatible for orthopaedic implants, various techniques are still used to enhance their properties. Different applications require specific properties even from the same material: in some cases implant is expected to stimulate bone ingrowth and increase attachment, but on the other hand inertness and low degradation rate may also be favourable in certain sections of the implant. Bone ingrowth and attachment can be enhanced by rough surfaces or textures on the implants. These surface conditions are achieved, for example, by different kinds of coating techniques or by etching. Smooth surfaces for preventing corrosion can be prepared by chemical or physical vapour deposition techniques. Amorphous diamond (AD) coatings can be produced by filtered pulsed arc discharge method which is one of the physical vapour deposition techniques. AD coating is reported to be even as hard as natural diamond, inert coating with low friction. Therefore, AD coatings should reduce the number of screw failures during installation as high torques leading to screw failures might be avoided. Also late removal of the screws is easier as less corrosion is expected.
Hip Implants
In total hip replacements, the bulk properties of materials, such as proper elasticity and hardness, are important. However, the material interacts with the body mainly at the surfaces. Wear and corrosion are initiated at the surfaces also. Therefore, the control of surface properties using different kinds of treatments or coatings may improve total hip replacements considerably. Until now, the surface treatments most studied included ion implantation and methods to control surface topography such as grit or sand blasting or plasma treatments. Among the large variety of coatings, hydroxyapatite, titanium oxide and nitride, zirconium oxide, pyrolytic carbon, and diamondlike carbon coatings have shown the most promising results. These coatings mainly are used to enhance bone growth; to minimize friction, wear, and corrosion; and to improve biocompatibility of total joint prostheses. The potential of novel coatings to solve some present problems in joint prostheses is discussed based on the structure and properties of different kind of coatings. It can be concluded that currently coating methods exist to improve the tribological performance and longevity of the total hip replacements. However, coatings must fulfill two essential requirements for successful long-term performance: no delamination in biochemical and biomechanical environments and sufficient protection of substrate from corrosion. These requirements have turned out to be an obstacle for development of reliable coating solutions for many medical applications.
Any implant material can be divided into two characteristic components: bulk material, which mainly is responsible for the mechanical and structural properties; and surface layer material, which interacts with the biologic environment. Throughout the history of total hip replacement (THR), the focus in material development has been the improvement of bulk properties of metals and, more recently, those of ceramics and polymers. For example, in the case of metals, minor improvements have been reached by alloying and novel processing techniques. Both of these approaches enhance the control of microstructure and composition, which are reflected in hardness and wear or corrosion resistance. For example, the yield strength of Co—Cr alloys have been improved from the typical values of cast alloys (450 MPa) to about 1350 MPa for high-temperature isostatic pressing alloys. Similar improvements have been achieved with Ti alloys and stainless steel, especially using mechanical deformation hardening. In the case of polyethylene (PE), the main achievement has been well-controlled cross-linking to enhance mechanical properties, especially wear resistance.
On the other hand, the properties of material surfaces have a major effect on interaction with tissues or components of modular prostheses. Novel techniques for surface treatment and deposition can be used to modify the surface of the implant, for example, to protect the implant from degradation and corrosion, to improve the surface structure or chemistry for tissue integration, or to increase wear resistance and to control friction at the interfaces. In most cases, these additional treatments can be used for existing clinically tested implant designs and bulk materials without adverse effects on component dimensions or bulk material properties.
Motivation for Coatings
Although bulk material properties have a major effect, for example, on elasticity or shock absorption properties of implants, properties of the surface layer are relevant for optimal behavior of each section of an implant. Low friction and wear rate are desirable at articulating surfaces and also at the other interfaces with micromotion between different components. Wear leads to release of debris particles to adjacent tissues, which can eventually cause aseptic loosening of an implant. Abrasive and adhesive wear are typical wear mechanisms. Abrasive wear (scratching) can be minimized using hard coatings or surface layers. Adhesive wear is related to sticking together of sliding surfaces, which can shear the softer material, in this case ultra-high molecular weight polyethylene (UHMWPE). In principle, adhesive wear could be minimized using materials with good wetting characteristics.
Different forms of corrosion in body fluids are a major concern especially in long-term clinical use. Modular THRs have several interfaces between different components, fixing materials, bone or synovial fluid for corrosion. Metallic elements used in orthopaedic implants are potential carcinogens or sensitizers of the immune system. However, either slight or no considerable risk for lymphoma or leukemia were found in patients who had a Co-alloy THR. On the other hand, the prevalence of metal sensitivity among patients with implant failure is approximately five times higher than the incidence in the normal population and two to three times higher than that of all patients with metal implants. Fortunately, different corrosion mechanisms could be avoided or at least diminished by biocompatible, high-quality, corrosion-resistant coatings. Although Ti alloys generally are considered to be nontoxic, they can be severely corroded because of crevice corrosion in an acidic environment, for example, at the stem-bone cement interface. Therefore, Ti alloys should be surface modified if used in contact with the bone cement or on articulating surfaces. As mentioned above, coatings can protect the implant against bone-cement fixation. In the case of press-fit acetabular cups and stems, in which the long-term fixation is based on bone ingrowth, implant surfaces can be manufactured to adapt better to bone using several methods such as meshing, texturing, and porous or bioactive coatings. Depending on the design strategy, these coatings either should be dissolved gradually or should provide a stable, nondissolving interface for bone growth. The properties of all the implant surfaces could be improved using coatings or surface treatments; however, this is not always necessary or commercially realistic.
Methods for Surface Modification
The idea of surface modification is to retain the desired bulk properties while modifying only the outermost surface, which interacts with the surrounding tissues and fluids or other components of the implants. Surface-modification methods, can be divided into two categories: (1) chemical, physical, or biologic modification of existing surface or surface layer; and (2) covering the bulk material with a material having different composition or microstructure. In principle, only the outermost molecular layers, that is the depth of about 1 nm, need to be modified or deposited. However, extremely thin layers easily are eroded or worn out, and therefore, the practical modified zone should be thicker, a few hundred nanometers or even several micrometers. This thickness can be compared with a typical dissolution rate of metallic implant materials in the body, about 50 nm/year.
Some typical methods used for surface modification of orthopaedic implants are summarized in the following. Because in most of the cases, the treatment or deposition process occurs at an atomic level, it is crucial to clean the implant surface before treatment using mechanical, chemical, plasma, or ion-beam methods. These methods remove contamination such as water vapor and hydrocarbons from the surface. In the physical vapor deposition (PVD) processes, deposition is carried out in vacuum and in most cases, energetic atoms, ions, or plasma (ionized gas) are used. Energetic ions make it possible to reach high local temperatures on ion impact or on the growing surface layer. Generally, this is advantageous because samples can be kept at low temperatures during treatment and still the films grow dense with a fine microstructure, leading to improved mechanical properties and corrosion resistance. Ion implantation is one of the best controlled PVD processes, in which accelerated ions with energies in the range of 10 to 106 eV are used to bombard the surface. With these energies, the range of ions varies from a few atomic layers to a few hundred nanometers. Ions can be formed from most of the atoms in the periodic table, and the energy and dose (total number of ions per unit area) can be determined accurately. Therefore, ion implantation has been used frequently for fundamental studies of the effects of dopant (added impurity) ions and for commercial applications in the electronics industry and also in biomedical companies such as Spire Corporation (Bedford, Mass.) and Implant Sciences Corporation (Wakefield, Mass.). Although ion beams often are used to improve corrosion or wear resistance, they can be used to modify polymer surfaces, too. Ion beams or plasma ion treatments can form, for example, nitrogen or oxygen functionalities, on the surface. These change important characteristics such as a hydrophobic polymer to a hydrophilic one, which enhances biocompatibility and wettability in biologic fluids. In a similar manner, hydrophobic properties can be achieved using higher-degree fluorinated compounds as a source for plasma ions.
Sputtering is one of the mostly used commercial processes to produce adherent films of metals, oxides, carbides and nitrides, even on large surfaces, at affordable prices. By proper selection of process parameters (gas pressure and composition, discharge, and bias voltage) thick, dense, fine-grained films can be deposited, which even can survive without delamination in corrosive body fluids and can withstand high surface pressures. Different variants of plasma techniques (dc, rf or laser plasma, plasma implantation, plasma etching, plasma arc, or pulsed plasma arc) are used to clean and etch the surfaces, to modify cell and protein reactions, to implant ions, to deposit coatings, and others. In fact plasma-surface modification is an effective and economical technique for many biomaterials and is of growing interest in biomedical engineering because of several advantages: reliability, reproducibility, nonline-of sight, sterile technique, relatively inexpensive, compatible with masking techniques to enable surface patterning, large selection of varied surface parameters, and others. Surface analysis is needed to ensure that the intended surface structures, compositions and properties really are achieved. Plasma-deposited films usually are almost free of voids and pinholes and show good adhesion to the substrate, which are remarkable advantages.
Chemical vapor deposition is based on the dissociation of gas molecules in a flow gas reactor to leave the desired atoms at the sample surface. Typically, high temperatures are needed and the coatings consist of large grains, which leave open corrosion paths for ions to the substrate. Because of these difficulties, chemical vapor deposition methods generally are not applied for orthopaedic applications. Microstructure of a coating is one of the most important parameters affecting the outcome of the coating in a biologic environment. The structure quite often is related to many functional properties of coatings such as hardness or corrosion resistance. In principle, the most perfect microstructure would be a single crystalline film, which is a fully dense, uniform structure where atoms or molecules are located in a perfect periodic structure. This kind of a structure is strong and can protect the substrate against corrosion, if the coating material is not dissolved at an atomic level. However, perfect single crystals cannot be grown on common implant materials. The next best solution is an amorphous structure such as amorphous diamond. This structure is less dense than a single crystal, but generally is uniform and smooth without open corrosion paths such as grain boundaries. Polycrystalline films, especially if they have oriented structures such as columnar grains, offer poor resistance against corrosion unless a corrosion resistant intermediate film is used between the substrate and the coating.
Delamination resistance of any orthopaedic coating is very important. High adhesion and delamination resistance can be achieved in different ways in deposition methods, for example, by covalent bonding between the substrate and the coating or surface layer, by intermixing layers or graded structures at the interface, appropriate functional groups for strong intermolecular adhesion etc.
The wear of PE in implants can be divided roughly into two components: adhesive and abrasive wear. Adhesive wear is characterized by the sticking of the polymer surface to a countersurface, leading to shearing of polymer material. This process produces micrometer-sized wear particles. In principle, wear is minimized by using wettable surfaces (higher surface energy, a water drop spreads easily). The wetting properties of surfaces and coatings such as amorphous diamond also can be tailored in a wide range using added impurities, for example, metals, F, K, P, and Ca. Abrasive wear (scratching) is caused by the surface roughness of the counterpart material and can be increased considerably by third-body abrasive particles or components of the lubricating fluid. With respect to both of these wear mechanisms, metals and ceramics behave fundamentally differently, especially in long-term clinical use. The ceramic oxides (alumina, zirconia) are more wettable than the metal surfaces, although a passivating oxide or oxyhydroxide film is formed on the surface of Co—Cr alloys, stainless steels, and Ti alloys. These passivating films are approximately 2 nm to 5 nm thick and are damaged easily by third-body wear particles or are sheared off. These third-body particles may derive from bone debris; polymethylmethacrylate; or oxide, carbide, or nitride particles from metal surfaces. The cyclic wear process, because of repeated shearing off and reforming of passivating film, generally is called oxidative wear. Although the typical average surface roughness of metal femoral heads initially is in the range 0.01 to 0.05 μm, the roughness of articulating metal surfaces increases with time because of the aforementioned reasons.
Ceramics and ceramic coatings are harder than normal third body particles in synovial fluid, such as, bone or bone cement. Therefore, they should remain smooth in long-term clinical use and the wear rate of possible soft counterpart material (UHMWPE) should not increase because of surface roughening. Indeed, based on the simulator experiments and clinical trials, ceramic surfaces remain stable and minimize long-term PE wear with typically 2 times to 4 times lower wear than with Co—Cr—Mo heads. In hard sliding pairs, ceramic-on-ceramic articulations are an enticing choice because of good wear resistance. In this combination, in addition to smoothness, an accurate fit of articulating surfaces is even more important than with pairs using PE. Current manufacturing techniques provide tolerances of surface sphericity better than 1 μm. These well-matched ball-cup pairs should allow hydrodynamic lubrication with a continuous fluid film. However, the clinical surveys have showed that articulating surfaces partly are in contact and that adhesive and abrasive wear occur. Therefore, in the case of identical sliding pairs, the materials should be as hard as possible to minimize wear. Even in this case, ceramic coatings could offer several advantages. For example, because of extreme hardness and good tribologic characteristics of diamond, continuous film lubrication is not needed in the case of amorphous diamond coatings. When the coating is thick enough (>20 microns), it can withstand high contact stresses and wear rate is negligible (less than 10 nm per 15 million cycles in a simulator). Furthermore, coefficient of friction is fairly low (<0.1) even in the early stages of an implant life cycle. Low friction is accompanied by low bending torque on fixation surfaces of the prostheses. The most important advantage of ceramic coatings compared with bulk ceramics, perhaps, is the fact that they are less prone to sudden complete failure, which is a feared, rare (less than 0.1%) complication of current ceramic-on-ceramic total hip sliding pairs.
In addition to articulating surfaces, wear occurs on all interfaces moving with respect to each other. For example, because of different elasticity of materials under cyclic loading, it seems impossible to achieve permanent rigid fixation of a stem using bone cement. Most of the stem designs allow movement (subsidence, micromotion) at the interface between bone cement and stem. However, the bone cement-bone interface is meant to be stable. Bone cements contain hard particles of ZrO2 or BaSO4, which are two agents commonly used to make bone cement radiopaque. These ceramic particles easily scratch any metal surface. However, hard ceramic coatings provide superior wear and corrosion resistance on the stem or cup surface against bone cement.
Protection Against Corrosion
In order to achieve good long-term clinical outcomes, the coatings must fulfill two essential requirements: they must be thick enough to withstand the high contact stresses against third-body particles and they must avoid delamination, which can be caused by corrosion through the pinholes of the coating. The process is stimulated when a surface layer contains micrometer-sized defects and especially if it is cathodic relative to its substrate Because of these reasons, poor long-term clinical results have been obtained, e.g. with TiN and oxide coatings. Surface roughness increases rapidly because of partial delamination of the coating, and the rough surface increases the wear rate of soft PE counterpart material.
In principle, surface treatments such as ion implantation, nitriding, or oxidation modifying the surface layer can be used to improve the hardness and corrosion resistance of metals and to reduce PE wear. In short-term laboratory tests, considerable improvements have been made because of the increase of surface hardness of metal. However, in long-term tests and in clinical use, the modified surface layer gradually becomes damaged by corrosion and hard third-body particles. For example, in the case of nitrogen-implanted Ti-6Al-4V total knee femoral components, the wear rate and roughness of bearing surfaces increased significantly (Ra even 1-2 μm) after only 1 to 3 million wear cycles, which is the same level as in the unimplanted case. In the case of similar Co—Cr alloy components, the surface removal rate was approximately 0.06 to 0.10 μm per 1 million cycles, and the possible advantageous effects of implantation, unfortunately, are assumed to be lost on articulating surfaces within a few years in vivo. Conversely, relatively thick, pin-hole-free ceramic coatings could be used on metal implants to provide inherent stability, to avoid long-term surface roughening, and to reduce wear of PE or cartilage. However, only a few successful results using the coatings for articulating surfaces have been published. Based on the reduction of polyethylene wear with zirconium oxide, titanium nitride or amorphous diamond coating on the counterface material even by 10 times to 50 times compared with uncoated Co—Cr femoral heads in laboratory tests, these coatings have great potential as a lifetime bearing combination.
One of the major problems in the use of hip and knee implants is the huge selection of different types of prostheses with a wide range of properties such as surface roughness, tolerances, and microstructural aspects leading to similar scatter in survival statistics. However, the well-functioning prostheses degrade, too. For example, Jacobs et al found that 3 years postoperatively, concentrations of implant metals in the serum and urine increased three-fold to eight-fold.
Functionalizing the Surface
In most of the applications discussed above, the role of the coating is quite passive from a biologic point of view, that is, it does not actively interact and enhance tissue function. However, low-temperature deposition processes such as surface-induced mineralization or sol-gel deposition can be used to attach proper functional groups on the surface in a series of self-assembled monolayers. Then, for example, a calcium phosphate coating can be grown from a liquid phase even on porous surfaces to get a rather uniform coverage. Growth factors to accelerate bone growth simply can be co-deposited with the coating at physiologic conditions.
Osseous integration effectively can be enhanced by polymer coatings such as poly d,l-lactide on pins or screws. Polymer coatings also can be deposited using simple methods such as dipping, and proper complexes of pharmaceutical agents such as growth factors can be incorporated in the polymer matrix. The polymer matrix and the amount of agents can be used to control the degree of aqueous diffusion into and out of the coating and drug solubility. Implant surfaces also can be covered by living cells, for example, by feeding sugar molecules for cells to cover their surface, which then can be used to attach cells on the implant surface and thus improve cell growth on the surface.
Ion, plasma and laser beams, and atomic-level mixing on the surface have been used conventionally to improve wear and corrosion of implant materials. However, they also can be used effectively to incorporate high amounts of calcium, phosphate, bioactive ceramics, or other species to reduce bone resorption and improve bone and implant integration. These techniques, in addition to photolithography, can be used to produce well-defined microstructural surfaces to promote cell attachment.
Laser-Ablation
In the recent years, considerable development of the laser technology has provided means to produce very high-efficiency laser systems that are based on semi-conductor fibres, thus supporting advance in so called cold ablation methods.
At the priority date of the current application, solely fibrous diode-pumped semiconductor laser is competing with light-bulb pumped one, which both have the feature according to which the laser beam is lead first into a fibre, and then forwarded to the working target. These fibrous laser systems are the only ones to be applied in to the laser ablation applications in an industrial scale. The recent fibres of the fibre lasers, as well as the consequent low radiation power seem to limit the materials to be used in the vaporization/ablation as the vaporization/ablation targets. Vaporizing/ablating aluminium can be facilitated by a small-pulsed power, whereas the more difficult substances to be vaporized/ablated as Copper, Tungsten, etc. need more pulsed power. The same applies into situation in which new compounds were in the interest to be brought up with the same conventional techniques. Examples to be mentioned are for instance manufacturing diamond directly from carbon (graphite) or alumina production straight from aluminium and oxygen via the appropriate reaction in the vapour-phase in post-laser-ablation conditions. When employing novel cold-ablation, both qualitative and production rate related problems associated with coating exist, thin film production as well as cutting/grooving/carving etc. has been approached by focusing on increasing laser power and reducing the spot size of the laser beam on the target. However, most of the power increase was consumed to noise.